1. Field of the Invention
The present invention relates generally to methods for hydraulically fracturing hydrocarbon bearing subterranean formations, and more particularly relates to methods for control of the vertical height of the fracture created in a subterranean formation by hydraulic fracturing procedures.
2. Setting of the Invention
In the completion of wells through hydrocarbon bearing rock formations, noncommercial wells often result because of low permeability to flow of hydrocarbons through the formation to the wellbore. This problem can be overcome by treating the formation in a manner designed to increase fluid flow toward the wellbore.
Hydraulic fracturing is a widely used well stimulation technique designed to increase the productivity of the well by creating fractures in the formation penetrated by the well to improve fluid flow through the formation. The technique normally involves injecting a fluid into the formation at a rate and pressure sufficient to propagate a fracture adjacent to the well. The fluid contains propping agents, termed proppants, for propping open the fracture and maintaining fluid conductivity through the fracture when the pressure applied during injection of the fracturing fluid is relieved.
During these hydraulic fracturing processes, however, it is often advantageous to confine the induced fracture to the particular formation being treated. It is therefore desirable that the fracture extend horizontally away from the wellbore with minimal growth of the fracture in a vertical direction. Confinement of the fracture is often achieved because of higher in-situ rock stresses in the overlying and underlying rock formations than the stresses in the formation being treated. However, during some hydraulic fracturing treatments, vertical height of the induced fracture occurs and the fracture grows out of the desired formation upward and/or downward. This vertical height growth can lead to a premature screenout of the treatment. A screenout occurs when the proppant becomes immobile at the leading edge of the fracture and prevents additional fluid injection and desired horizontal extension of the fracture. Vertical height growth into an adjacent water zone can affect subsequent production of desired hydrocarbons from the well. Moreover, vertical height growth increases the amount of fracturing fluid needed to achieve the desired horizontal extension, thus increasing costs of a treatment. Consequently, techniques for control of the vertical height of the induced fracture during hydraulic fracturing treatments are important to prevent waste, inefficient extension, and growth into undesirable adjacent zones.
One such technique for height control during hydraulic fracturing is proposed in Cleary "Analysis of Mechanisms and Procedures for Producing Favorable Shapes of Hydraulic Fractures," SPE 9260, 55th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, Sept. 21-24, 1980. Cleary describes a technique for height control using "heavy/light particles, mixed with the frac fluid" which settle and rise to the bottom and top of the fracture and reduce the flow transmissivity where the particles congregate. FIG. 5 of the Cleary article shows the heavy/light particles are proppant and buoyant beads.
U.S. Pat. No. 3,335,797, "Controlling Fractures During Well Treatment," issued to F. H. Braunlich, Jr., on Aug. 15, 1967, claims a procedure for hydraulic fracturing to create "a fracture pattern which may progress to a greater extent outwardly and upwardly and to a lesser extent downwardly." Braunlich employs a particulate propping agent of particle size "between about 20 and about 60" to "pack together sufficiently to divert subsequently injected liquids but retain some permeability. "
To Applicant's knowledge, however, it is previously undisclosed to inject during a hydraulic fracturing treatment a nonproppant fluid stage which denies fluid flow into the vertical extremities of the fracture, and thus controls vertical height growth in both uphole and downhole directions.